Research On Arc Stability And Welding Process Of Ultrasonic Assisted Underwater Wet Welding

Base on conventional underwater wet welding and ultrasonic assisted welding, ultrasonic assisted underwater wet welding(U-FCAW) method was developed in this paper. With the application of ultrasonic, the arc stability was improved and the microstructure and mechanical properties of weld increased. First, the mechanism of ultrasonic effects on U-FCAW arc bubble was investigated through the simulation and experiments. Then, the arc stability of U-FCAW was explored in detail. Finally, the comparison of U-FCAW and FCAW was conducted to reveal the ultrasonic effects on weld microstructure and mechanical properties.The ultrasonic model was built based on COMSOL Multiphysics. The ultrasonic field of different radiation height was calculated. The results show that an upward ultrasonic force was formed on the workpiece surface. The amplitude of ultrasonic pressure reached the maximum value at the resonant radiation height. The ultrasonic field was changed because of the bubbles. A downward ultrasonic force was formed above the bubbles while there was no effects on the bubbles inside. The sound pressure measurement experiments was conducted and the results were coincides with the simulation results. According to the analysis of the forces on the bubble, an additional force, which was named as ultrasonic force, was formed in U-FCAW, which reduced the rising velocity of the bubbles produced in the welding process.The arc stability of U-FCAW was also studied. The results showed that there are three regions, i.e., short-circuit area, arc extinction area and arc burning area. With the voltage increased, arc extinction and short-circuit time were decreased, the coefficients of variation of voltage and current were decreased. When the voltage increased to 34 V, few arc extinction was discovered in the U-FCAW welding process. With the assisted of ultrasonic, short-circuit frequency was decreased, the arc stability was increased. Droplet force analysis showed that with the assist of ultrasonic, the number of the forces acting on the molten drop was keep constant, while the value of forces changed. The surface-tension force decreased as a result of the reduction in metal transfer frequency. The forces on the bubble changed with the assist of ultrasonic, a downward ultrasonic force can restrain the floating of the bubble and decrease the adverse perturbation of bubble burst, which can increase the arc stability.Finally, the effects of ultrasonic on the weld microstructure and mechanical properties were obtained. The content of martensite(M) and upper bainite(BU) decreased, while the granular bainite(BG) and acicular ferrite(AF) increased with the ultrasonic assisted. The fracture behavior of weld joint tensile test was transformed from joint to base metal, compared with traditional underwater wet welding. Tensile strength of upper and lower layer increased from 338 MPa and 267 MPa to 495 MPa and 395 MPa, respectively. The maximum angle of bend test was increased from 21 deg to 84 deg.